Conventionally, boilers belonging to thermal power plants employing, as a fuel, petroleum, coal, fuel gas, or the like, as well as various large-scale boilers, waste incineration apparatuses, and other apparatuses are generally equipped with a flue gas NOx removal apparatus. Such a flue gas NOx removal apparatus includes therein a plurality of NOx removal catalyst layers.
A typical NOx removal catalyst contains a carrier (e.g., TiO2 or the like), an active component (e.g., V2O5), and a co-catalyst component (e.g., tungsten oxide or molybdenum oxide). These catalyst components are used as a complex oxide such as VOx—WOy—TiO2 or VOx—MoOy—TiO2.
Such an NOx removal catalyst is generally in the form of a honeycomb-shape catalyst or a plate-like catalyst. Examples of the honeycomb-shape catalyst include a coating-type catalyst produced by forming a honeycomb-shape substrate and coating the substrate with catalyst components; a kneading-type catalyst produced by kneading a catalyst substrate with catalyst components and forming the mixture into a honeycomb; and an impregnation-type catalyst produced by impregnating a honeycomb catalyst substrate with catalyst components. The plate-like catalyst is produced by coating a metallic core or a ceramic substrate with catalyst components.
In any case, when such an NOx removal catalyst is used continuously, a substance which deteriorates the performance of a catalyst (hereinafter referred to as a deteriorating substance) is deposited on or dissolved into the catalyst at the surface and inside thereof, thereby incidentally impairing the catalytic performance.
In order to solve the problem, studies have been conducted on a method of regenerating the NOx removal catalyst.
The conventionally studied countermeasures are based on physically removing deteriorated parts and contaminants, to thereby cause the active surface to be exposed. Specific examples include a technique in which the walls of exhaust gas flow paths are polished with an abrasive (see Patent Document 1), a technique in which a surface portion of a deteriorated NOx removal catalyst is ground, to thereby renew the catalytically active surface (see Patent Document 2), and a technique in which a gas containing microparticles is caused to pass through the through-holes of a catalyst, to thereby remove contaminants (see Patent Document 3).